Photochromic display systems are known which are based upon the spectral energy absorption characteristics of certain photochromic materials. In such systems, the alpha-numeric characters or other elements of a scene are written as a photochromic image onto a photochromic film and are viewed on a screen simultaneously with details on a map slide. Such a system is described in U.S. Pat. No. 3,706,487 of S. F. La Forgia, issued Dec. 19, 1972.
Previously a suitable, convenient, large screen, dynamic real time color display has not been achieved. A real time display is desired which is capable of obtaining color images from a single frame of photochromic film, since multi-image plane techniques yield unsatisfactory displays. A typical prior art display uses laser writing techniques whereby friendly positions are represented by blue symbols having a border of a single line, while enemy positions are seen as blue symbols having a dual-line border.
In accordance with this invention, a large screen, dynamic real time, single image plane color display has been devised using a two-layered photochromic film technique such as will be described subsequently. Although this technique does not allow for a full color dynamic display, a two-color display has advantages, particularly in military applications which rely on the use of only four colors, viz., green, yellow, red, and blue. Green is used for man-made barriers and obstacles, both friendly and enemy, and yellow is used for areas of nuclear fallout or bacteriological contamination, both friendly and enemy. Since obstacles and contamination areas are written symbolically anyhow, little is gained by having them appear green and yellow, and, in fact, too many colors on an already congested military map could be confusing. Having the color blue for friendly personnel and red for enemy personnel, however, offers good differentiability so that commanders can more quickly and correctly assess the military situation and make more timely and accurate command and control decisions. Personnel using the display can readily distinguish enemy positions (red symbols) from friendly positions (blue symbols). A third color can be obtained by writing the same information in both red and blue, thereby yielding a magenta color. By permitting projection of two colors (blue and red), one does not require, as in the prior art display mentioned above, that part of a computer memory for double line enemy position symbol generation and storage. Consequently, friendly and enemy positions are much more easily distinguishable from one another.
The character information is written selectively onto either a blue photochromic layer on one side of the thin film or onto a red photochromic layer on the other side of the thin film. Information-carrying ultraviolet radiation is selectively directed over one of two paths, depending on which of two color symbols are to be written and projected. The ultraviolet light, upon reflection from a selected mirror, is made to impinge upon a corresponding one of two dichroic mirrors to reflect the ultraviolet light onto a corresponding one of the two photochromic layers, and thereby light either a red or a blue character on that selected photochromic layer. Simultaneously, one portion of a projection beam from a light source, after removal of any infrared and ultraviolet components therefrom, is transmitted through said corresponding dichroic mirror, the selected character bearing photochromic layer, the film and the other dichroic mirror. By suitable optics, the information is projected to the position of a transparent map overlay, and thence the map and dynamic characters are projected by means including a condensing lens and projection lens onto a viewing screen. In other words, as the color characters are being selectively written, they are simultaneously projected on this screen. The other portion of the projection beam from the optical source is transmitted through a separate lens system for recombination, just ahead of a condensing lens, with said one portion (character-containing portion) of the projection beam which passes through the selected photochromic layer.
In summary, a two-layer thin film can be written on from both sides, thus allowing two color symbols to be written in a substantially single image plane, while, at the same time that the colored symbols are being written, they are being projected onto a large screen for viewing, thereby providing a real time dynamic display. In addition, the two color technique of the invention has the added capability of ready incorporation into presently available monochromatic systems with far fewer problems then would be encountered when implementing techniques already used.